Ferrules are secured to the ends of optical fibers to provide protection, and to enable the ease of use of optical fiber cables for the transmission of information via laser light energy. Ferrules provide the means for make-break connections, and to terminate the optical fiber transmission cables to sources, detectors, or to connect to other optical fiber cables. Optical fibers can be divided into two groups, the first, a multimode optical fiber that transmits laser energy of multiple laser modes, and the second, a single mode optical fiber that transmits a single laser mode or frequency. The multimode optical fiber is more easily connected to another optical fiber as the core of the multimode optical fiber is larger and tolerances to match two multimode optical fibers is much less restrictive than it is to connect two single mode optical fibers where the core of the optical fiber is much smaller. Thus the single mode optical fiber needs to be much more precisely aligned in order to obtain the equivalent transmission losses when compared to a multimode optical fiber. However, a single mode optical fiber is preferred over a multimode fiber when large amounts of information need to be transmitted.
Ceramic has been the preferred material for ferrules used with a single mode optical fiber. Ceramic ferrules can be made with tight tolerances on diameter, roundness, and concentricity of the location of the hole to hold the optical fiber. Currently, the method for manufacturing a ceramic ferrule includes the forming of the ceramic ferrule in the green state, firing the ceramic, lapping or polishing the hole to a given diameter within certain tolerances, grinding or lapping the outer diameter to within certain tolerances of diameter and concentricity of the axis of the hole all of which is time consuming and expensive. Often to achieve tight tolerances, ferrules are individually selected to match a particular optical fiber. The ceramic material has many beneficial properties. It is tough, hard, can be polished, has excellent wear resistance for insertion into sockets. However, it also has some limitations. The hole for the optical fiber is fixed and since optical fibers also have a range of diameters, due to tolerances of manufacture, only optical fibers with a smaller diameter than the hole in the ferrule can fit inside a given diameter hole in a ceramic optical ferrule. The difference between the optical fiber diameter and the diameter of the hole in the ceramic ferrule will add to the eccentricity of the optical fiber from the center axis of the ceramic ferrule, resulting in greater loss of light energy in connections. This has been partially overcome by selecting ceramic ferrules to match optical fibers. Other approaches have been used; however, all result in additional manufacturing steps and increased cost to insure low loss connections of optical fibers.
Thus a need exists for an optical ferrule that can be easily manufactured and requires no individual selection to achieve a low loss optical fiber connection. Injection molding processes offer ease of manufacturing. Patents which illustrate molded plastic ferrules include U.S. Pat. No. 4,330,171 and U.S. Pat. No. 4,942,009. Also see, for example, U.S. Pat. No. 4,531,702 which describes a three-part mold set for which concentricity and axial location of the mold parts is effected by directing a mold closing force along the common axis of the parts. Injected molded ferrules have not been uniform enough in roundness, or concentricity to be used as low loss optical ferrules for single mode optical fibers. Also, some of the other traditional problems of injection molding have also been hard to overcome, such as part shrinkage that results in large variations in dimensions. However, previous attempts to make optical ferrules have not resulted in tight tolerance ferrules that are sufficient to be used for low loss single mode optical fiber connectors.
The present invention overcomes many of those limitations and provides an injection molded plastic ferrule retaining most of the advantages of a ceramic ferrule, and providing some additional benefits at reduced cost. Thus, the purpose of the present invention is to provide an injection molded plastic ferrule for use with single and multimode optical fibers that requires tight tolerances on diameter, roundness, and concentricity of hole to outer diameter for use with a typical manufactured variance of diameter of optical fibers. Another purpose is to provide a pigtailed optical fiber ferrule composition with tight tolerances on diameter, roundness, and concentricity of the center of the optical fiber to the diameter of the ferrule that can be polished in a "factory" and used in a connector assembly for field installation to the end of an optical fiber cable via a splicing device in the assembly to insure dependable low loss field installed optical fiber connectors.